Short-Range Order in VI3

Terahertz Magnetic and Lattice Excitations in van der Waals Ferromagnet VI3

We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI₃. The structural transition at T_S1 = 79 K is accompanied by a large splitting of polar phonon modes. Below T_S1, strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below T_C = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the temperature T_S2 of the second structural transition associated with the order-order magnetic phase transition. Below T_S2, a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI₃ indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has previously been considered the exclusive domain of antiferromagnets.